Various forms of apparatus including one or more vibrating elements are well is known in the field of plant or process control, one example being a contact fluid level transducer. A typical contact fluid transducer includes a pair of spaced tines mounted on a diaphragm, the diaphragm in turn defining one end of a hollow cylindrical body. The tines are arranged to vibrate at a given frequency, typically their resonant frequency, and vibration is typically effected by displacing the centre of the diaphragm by means of a compressed stack of piezoelectric elements, located within the hollow body, and driven by a cycling voltage. When the vibrating tines are contacted by a fluid, there is a change in the frequency at which they vibrate. By detecting the change in frequency, one can determine when a rising fluid level contacts the tines. Likewise one can detect when a fluid level drops below the level of the tines.
An early example of this type of level detection transducer is described in UK Patent No. 2 150 292. In this device a stack, consisting of piezoelectric elements, insulators and connectors, is compressed against the inner or reverse side of a diaphragm by a compression screw mounted in a bridge piece extending over that end of the stack remote from the diaphragm. The spaced tines extend from the outer or front side of the diaphragm, which diaphragm forms one end of a hollow body in which the piezoelectric stack is located.
The bridge piece is, in turn, mounted on a pair of spaced rod-shaped supports extending from the inner side of the diaphragm and also located within the hollow body.
When the piezoelectric elements in the stack are subjected to a cyclic drive voltage, the same expand and contract between the compression screw and the diaphragm, deforming the diaphragm and causing the tines to vibrate. A problem with existing designs is that, at point of manufacture, the stacks are assembled piece-by-piece into the apparatus. This process is somewhat labour-intensive and, once assembled, the apparatus does not lend itself to repair, particularly on-site repair, in the event of failure in the piezoelectric stack assembly. In the event of failure of any component in the piezoelectric stack, the entire apparatus is either removed and replaced, or removed and returned to a service facility for repair. Whichever the case, the seal between the transducer and the plant must be broken, and the transducer removed from its operating environment. In a significant number of cases, apparatus of this type are installed in industrial plants which are subject to stringent inspection and control. If a device is removed for any reason, when that device is replaced or re-installed, the plant may have to be inspected and approved before normal operation can be re-established. A further problem is that the plant served by the transducer may have to be drained before the transducer can be removed. Whatever the case, transducer failure inevitably causes delay and expense, and may also give rise to health and safety issues.
It is an object of the present invention to address the above mentioned problems; or to provide a method and/or apparatus applicable to vibrating element apparatus, which will at least provide a novel and useful choice.